1. Field of the Invention
This invention relates to navigation and positioning systems, and more particularly to a system and method for calculating highly accurate heading information through the use of multiple GNSS receivers separated by a short distance.
2. Description of the Related Art
Systems for determining the heading of a moving body, such as a vehicle or a person, exist in the prior art. One such system is a magnetic compass, which is a navigational instrument for determining direction relative to the Earth's magnetic field. It typically consists of some form of freely-rotating magnetized pointer that can align itself with the Earth's magnetic field. However, solid state versions of the magnetic compass also exist, wherein solid state magnetic sensors are used to sense the compass's orientation relative to the Earth's magnetic field. In its original form, the magnetic compass is one of the oldest and least complex navigational instruments in existence.
Unfortunately, a magnetic compass is subject to several limitations. A magnetic compass may be affected by the presence of large amounts of metal, particularly ferrous metals used in a ship's hull or in the body of a ground vehicle or aircraft. A magnetic compass may also be skewed by large deposits of iron ore in the ground, such as that present in the Marquette Iron Range in Marquette County, Michigan.
Non-magnetic compasses have been developed to try to overcome the limitations of magnetic compasses. Gyro-compasses employ a fast-spinning wheel and the forces of friction to determine heading based on the rotational axis of the Earth. Although any gyro can act as a gyro-compass, because the earth's rate of rotation is small, very accurate gyroscopes such as fiber-optic gyroscopes (FOG) or laser ring gyroscopes are desirable. The gyro-compass can determine when it is aligned with the rotational axis of the Earth, and therefore provides true north as opposed to magnetic north. Because the operation of a gyro-compass depends on the rotation of the Earth, it will not function correctly if the vehicle or body to which it is mounted is moving fast in an east to west direction. The gyro-compass can be subject to errors caused by rapid changes in course, and the gyro-compass is subject to drift because of inertial effects on its mechanical parts and electrical measurement imperfections.
To avoid the problems of drift caused by mechanical parts, some systems employ a ring laser gyroscope or fiber optic gyroscope. A ring laser gyroscope has no moving parts, but instead relies upon beams of laser light bouncing around a ring of mirrors in a device. The beams are channeled to a photo-detector. If the vehicle to which the ring laser gyroscope is mounted is not changing heading (not rotating), the light beams will remain in phase. If the vehicle or body is rotating, however, one of the beams will change phase with respect to the other. A fiber optic gyroscope works on a principle similar to the ring laser gyroscope, but the light is channeled through fibers.
However, ring laser gyroscopes and fiber optic gyroscopes are very expensive systems and therefore do not work well for all applications. In particular, building a head- or human-mounted heading determination system based on a ring laser gyroscope or similar system would not be practical or affordable.
Global navigation satellite systems (GNSS) such as the Global Positioning System (GPS) can be used to provide a form of heading when a vehicle or body is in motion. For example, if a single GNSS receiver can derive its position at one point in time, and then derive a second position at a point in time one second later, it can be assumed that the GNSS receiver moved from the first location to the second along a heading defined by a line from the first point to the second point.
However, GNSS-based systems using a single receiver and single antenna do not work when the vehicle or body is not moving. For example, if a helicopter is hovering in one spot (one GNSS location) for a period of time, there is no way to determine what direction that helicopter is currently pointed, as there is no movement from one spot to another with which to determine heading. Similarly, a head- or human-mounted GNSS system with a single receiver and single antenna cannot determine the human's heading when the human is standing in one spot. There is no way to tell which way the human is looking if they are not moving.
Prior art systems describe the use of two GNSS antennas separated by a short distance in an attempt to overcome the limitations of single-antenna GNSS systems. These two-antenna GNSS systems measure the difference in phase of a satellite signal as it is received by two antennas to attempt to derive the position and orientation of the system. However, these two-antenna systems are typically intended to be mounted in a known orientation and are subject to the affects of roll and pitch as the vehicle or body to which the system is mounted moves in space. It is very difficult to obtain a highly-accurate heading using a two-antenna GNSS system unless there is some mechanism for compensating for roll and pitch and other movements.
What is needed in the art is a highly-reliable heading determination system which uses at least two GNSS receivers with at least two corresponding GNSS antennas separated by an ultra-short baseline (typically less than 0.5 meters), coupled to an inertial measurement system to compensate for roll and pitch and which can be used on a body where the orientation of the body in space is highly dynamic, but the GNSS location of the body may be static for long periods of time.